This invention relates to a method of manufacturing an image sensor by arranging a large number of photoelectric converting elements on a substrate, and, more particularly, to an image sensor manufacturing method which effectively reduces a leak current generated between the adjacent photoelectric converting elements.
To date, various types of image sensors have been accepted tor use in facsimile units, optical character readers, electronic image pickup tubes, etc. Attention has been drawn particularly to a long, one-dimensional image sensor having the same pickup length as the width of an original impression. Energetic headway is being made in the research and development of image sensors. In recent years, great demand has arisen, in the interest of ensuring high resolution reading, for a high density arrangement of photoelectric converting elements.
Among the desirable properties of a photoelectric converting element involved in an image sensor, the degree of resolution constitutes one of particular importance. The conventional image sensor is accompanied with the drawbacks that, as the photoelectric converting elements are more closely arranged, a larger leak current is generated between the electrodes of the adjacent photoelectric converting elements. Consequently, sufficient improvement in resolution is not ensured, even if the photoelectric converting elements are densely arranged.
Detailed description may now be made of the above-mentioned problems. With a long image sensor, a plurality of photoelectric converting elements are generally arranged on a substrate. The photoelectric converting element is constructed by arranging separate electrodes prepared from, for example, chromium, on a substrate; depositing an amorphous semiconductor layer 4 such as that prepared from amorphous hydrogenated silicon (a-Si:H) on said separate electrodes and substrate; forming a common electrode prepared from a transparent conductive layer prepared from, for example, indium tin oxide (ITO); and interposing said amorphous semiconductor layer between the separate electrodes and common electrode (a sandwich arrangement). In the above-mentioned sandwiched arrangement, photoelectric current I.sub.P, conducted through the respective photoelectric converting elements by incoming light rays, involves not only signal current I.sub.C which has a directional flow due to the presence of a potential barrier produced in an interface between the amorphous semiconductor layer and common electrode, but also leak current I.sub.L which has no directional flow due to the presence of a potential difference between respective individual electrodes 3-1, 3-2 and 3-3. Said leak current I.sub.L reduces the resolution degree of an image sensor.
With respect to image sensors comprising photoelectric converting elements each being composed of ITO/a-Si:H/Cr, and arranged with densities of 2/mm to 24/mm, determination was made of the relationship between the ratio (I.sub.P /I.sub.C) of photoelectric current I.sub.P and signal current I.sub.C, and the potential difference between the adjacent individual electrodes, the results being set forth in FIG. 1. The above-mentioned data were obtained under conditions in which the temperature was set at 20.degree. C. and the luminosity of the light beams illuminated on the image sensors was 100 luxes. The data of FIG. 1 shows that when a potential difference between individual adjacent electrodes stands at 0.5 volts, the ratio of the leak current to the signal current estimated at 30% when the image sensors are arranged at a density of 8/mm, 60% when the image sensors are arranged at a density of 16/mm, and 120% when the image sensors are arranged at a density of 24/mm, indicating that the upper limit of high density arrangement of image sensors is 8/mm. When image sensors are arranged at a higher density than 8/mm, it is necessary to suppress the occurrence of a leak current by some means or other.
To date, the undermentioned processes have been adopted to suppress the occurrence of a leak current: (1) A highly resistive layer has been applied as a photoelectric converting layer. (2) A light shield has been provided to shut out unnecessary light beams brought into the regions defined between the adjacent photoelectric converting elements. (3) The respective photoelectric converting elements have been electrically isolated from each other. Process (1) however, has the drawbacks that the properties of the image sensor are harmfully affected by the nature of the photoelectric converting membrane, with .mu..tau. (a factor representing carrier mobility.times. carrier life) in particular dropping, resulting in a decline in photo-responsivity and band characteristics. Similarly, processes (2) and (3) are accompanied by the difficulties and high cost that, during the manufacture of an image sensor, it is necessary to make the photo etching process